In bidirectional radio systems of mobile communication networks, the transmitting and receiving bands are relatively close to each other. In the full duplex system, in which signals are transferred in both directions simultaneously, it must be especially ensured that a transmitting of relatively high power does not interfere in the receiving or wide-band noise of the transmitting block the receiver. The output signal of the transmitter power amplifier is therefore strongly attenuated on the receiving band of the system before feeding to the antenna. When the transmitting band is above the receiving band, a high-pass filter is sufficient for that in principle. However, if signals of some other system, the spectrum of which is below the above mentioned receiving band, are also fed to the antenna through the same antenna filter, a band stop filter is needed for the attenuation.
FIG. 1 shows an example of a known band stop filter used as an antenna filter. The filter 100 comprises, in a unitary conductive filter housing a first R1, a second R2 and a third R3 coaxial resonator, which have no mutual coupling. The filter housing has been drawn in FIG. 1 with its cover removed and cut open so that the inner conductors of the resonators, such as the inner conductor 101, are partly visible. The inner space of the housing is divided by conductive partition walls into resonator cavities. The lower ends of the inner conductors of the resonators join galvanically to the bottom of the housing and thus to the signal ground GND. Their upper ends have only a capacitive coupling to the cover of the housing and the surrounding, conductive walls, and so the resonators are quarter-wave resonators. In addition, the filter 100 comprises a coaxial transmission line 120 and an arrangement for coupling the transmitting line to the resonators. The transmission line runs through three coaxial T-connectors, which are galvanically fastened to one side wall 112 of the resonator housing. The first T-connector 131 is at the first resonator R1, the second T-connector 132 at the second resonator R2 and the third T-connector 133 at the third resonator R3. In the example of FIG. 1, the electric distance between two successive connectors is a quarter of the wavelength on the middle frequency of the filter stop band, which is an advantageous length with regard to the matching of the transmitting path. The conductive casing of the branch part of each T-connector is in galvanic contact with the side wall 112, and so the outer conductor of the transmission line becomes connected to the ground GND. The inner conductor of the branch part of the first T-connector has been connected to the first coupling element 141 in the cavity of the first resonator. That element is a rigid conductor, which in this example extends relatively close to the upper end of the inner conductor 101 of the first resonator. In this way, the first resonator becomes electromagnetically coupled parallel with the transmission line 120. In the same way, the second resonator becomes coupled parallel with the transmission line by means of the coupling element 142 in the cavity of the second resonator, and the third resonator by means of the coupling element 143 in the cavity of the third resonator. The shape of the coupling element can vary, and it can be, for example, a loop conductor going round the lower end of the inner conductor of the resonator.
The ends of the transmission line 120 function as the input and output ports of the band stop filter 100. The end of the transmission line on the side of the first resonator is, for example, the input port IN and the second end is the output port OUT. The band stop property is based on that the resonator represents at its natural frequency a short circuit as viewed from the transmission line. In that case the energy fed to the transmission line is almost entirely reflected back to the feeding source, and hardly any energy is transferred to the load coupled to the output port. At frequencies that are clearly lower or higher than the natural frequency, the resonator is seen as a high impedance, in which case the energy of the signal is transferred to said load without any obstacle. One resonator provides a relatively narrow stop band. By using more than one resonator and by adjusting their natural frequencies to have different values but suitably close to each other, the stop band can be widened.
FIG. 2 shows two examples of the amplitude response of a three-resonator band stop filter. The response curves 21 and 22 show the change of the transmitting coefficient S21 of the filter as a function of frequency. The smaller the transmitting coefficient, the higher the attenuation of the filter is. In both cases, the natural frequencies of the resonators have been arranged at the points 1925 MHz, 1950 MHz and 1975 MHz, for which reason an attenuation peak occurs at these frequencies. Between two adjacent attenuation peaks, the attenuation gets a minimum value, which is the minimum attenuation in the stop band, or more briefly, the stop attenuation. The attenuation values depend on the strengths of the electromagnetic couplings arranged by the coupling elements in the resonators. In the case of the first curve 21, the stop attenuation is arranged to the value 20 dB by the coupling elements, and to the value 40 dB in the case of the second curve 22. It can be seen from the shape of the curves that increasing the attenuation widens the transition bands of the filter. A transition band means an range between the stop band and the pass band, when the pass band is considered to be an range on which the attenuation is, for example, 1 dB at the highest. In duplex systems, the range between the transmitting and receiving bands, or the duplex spacing, has been specified to have a certain value. The transition band of the filter must naturally be narrower than the specified duplex spacing, which means that the stop attenuation cannot be freely increased. This also applies to filters according to the invention.
One drawback of the filter according to FIG. 1 is a relatively large number of structural parts in the transmission line structure, which increases the production costs. A large number of parts also means numerous conductive junctions, which causes harmful intermodulation. Where a transmission end filter is concerned, the problem is emphasized because of the relatively high currents that occur in it. A further drawback is the difficult tuning of the filter. The tuning includes both setting the natural frequencies of the resonators and setting the strengths of the couplings between the resonators and the transmission line. In accordance with the above-described, the tuning takes place by bending straight coupling elements or by shaping loop-like coupling conductors in relation to the inner conductors of the resonators. The resonators are not entirely isolated in practice, but the tuning of one influences the natural frequencies of the others through the transmission line of the filter. This results in a number of manual iteration rounds in the tuning, which means a significant cost factor in production.